SUMMARY: Prostatic tissue from young mature dogs was incubated in vitro with [4-14C]testosterone to assess androgen metabolism by this target organ. At the same time tissue from a dog previously treated with implants of oestradiol-17β and tissue from an older animal were also studied. Tissue from the oestrogen-treated dog showed considerable qualitative and quantitative differences from the normal in regard to metabolic activity. Fractionation of prostatic tissue after incubation with labelled testosterone indicated the localization of certain enzymes and products in the nucleus, and 5α-androstane-3α,17α-diol formation appeared particularly interesting. The relative activities of testicular tissue from the dogs in regard to the formation in vitro of androstenedione and testosterone from various radioactive precursors were measured.